Population ecology of the barnacle Chthamalus stellatus in the northwest Mediterranean

This study examined patterns in the distribution and demography of the barn acle Chthamalus stellatus (Poli) at different spatial scales in the northwe st Mediterranean. Preliminary data indicated that the abundance and size of barnacles decreased from high-shore to low-shore habitats. The generality of these patterns was investigated at several locations (10s to 100s of km apart), at several sites within locations (100s to 1000s of m apart) and at different times, patterns were consistent with the preliminary observation s, despite considerable spatial and temporal variability at small and large spatial scales. The following models were proposed to explain the observed patterns: (1) recruitment was intrinsically greater high on the shore, (2) limitation of recruitment due to pre-emption of the substratum was greater low than high on the shore, (3) environmental conditions reduce growth low on the shore, and (4) mortality was greater low on the shore. The predicti ons of these models were tested by examining patterns of recruitment, growt h and mortality of barnacles and availability of free space in relation to height on the shore, at several spatial scales and through time. Successful recruitment of barnacles was observed at different heights on the shore wh ere resident organisms were removed, despite a trend toward a larger number of recruits high on the shore at 1 location (Livorno). Availability of bar e rock for recruitment was greater high on the shore, implying that pre-emp tion of the substratum was more intense low on the shore. There was no evid ence to suggest that barnacles grew faster on the high shore than on the lo w shore. The opposite pattern was observed for young barnacles in several c ases. Mortality rates were generally greater in low-shore than high-shore h abitats for young and for adult barnacles. Patterns emerged over a backgrou nd of considerable spatial and temporal variation. These results emphasized the importance of pre-emption of space and mortality of juveniles in gener ating patterns in the distribution and structure of populations of barnacle s on rocky shores in the northwest Mediterranean.